Conventionally is known graphitic cast steel which contains precipitated graphite nodules therein for improving the properties of the cast steel for plastic working and machining. As well known in the art, it is preferable to have fine graphite nodules distributed in the cast steel as densely and uniformly as possible in view of improving frictional and machining properties characterized by short machining chips.
As a method for precipitating graphite in cast steel, it is conceivable to use a heat treatment process, but a desired result may not be obtained because the heat treatment will require a considerable time period and the precipitated graphite may be too coarse to be acceptable and may not be as nodular as desired.
For instance, in Japanese patent laid-open publication (kokai) No. 63-103049, it is disclosed to add rare earth elements for the purpose of distributing fine graphite nodules in the cast steel at high density and uniformity. This non-examined patent publication teaches that machining property can be improved by adding 0.4 wt % or less of bismuth as an element for improving machining property (0.02 wt %, 0.05 wt % and 0.13 wt % in the disclosed embodiments), and that as the bismuth content increases beyond 0.4 wt % the graphite will lose the nodular shape and both machining and mechanical properties will be impaired.
However, the Inventor has discovered that, even in the above mentioned technology, fine graphite particles are favorably dispersed only when the cooling rate at the time of casting is sufficiently high, and chain-shaped graphite formation tends to develop in the manner of networks when the cooling rate is low due to the large size of the product or the nature of the process of casting, depending on the content of bismuth. Even within the same product, the parts involving low cooling rate such as thick wall portions and the sprues tend to be subjected to such problems. Such chain-like formation of graphite crystals impairs such properties as mechanical strength, elongation and rigidity, or results in substantially undesirable mechanical properties in the cast steel as opposed to favorably distributed graphite nodules.